Time constant automatic adjusting circuit, filter circuit system, and method of automatically adjusting time constant

Abstract

A time constant automatic adjusting circuit comprises: a filter circuit varying a phase of an clock signal to be input so as to output the clock signal; a phase comparison circuit comparing a phase of an output signal of the filter circuit with the phase of the clock signal, and outputting a predetermined voltage when the phase of the output signal and the phase of the clock signal are the same; at least three comparators comparing the output voltage with a plurality of different voltages; an up-down counter counting a number of output bits of either one of the at least three different voltages in accordance with an output result of the comparators; and a control circuit varying the time constant of the filter circuit in accordance with the number of output bits counted by the up-down counter.

What is claimed is: 
     
       1. A time constant automatic adjusting circuit comprising:
 a filter circuit capable of varying time constant, the filter circuit varying a phase of a clock signal to be input so as to output the clock signal; 
 a phase comparison circuit comparing a phase of an output signal of the filter circuit with the phase of the clock signal, and generating output voltage according to a phase difference between the phase of the output signal of the filter circuit and the phase of the clock signal; 
 at least three comparators comparing the output voltage with a plurality of different voltages; 
 an up-down counter counting a number of output bits of either one of at least three different values, each values being set in accordance with an output result of the comparators; and 
 a control circuit varying the time constant of the filter circuit in accordance with the number of output bits counted by the up-down counter; and 
 a switch capable of connecting or disconnecting the clock signal to be input to the filter circuit and the clock signal to be output from the filter circuit to the phase comparison circuit. 
 
     
     
       2. A time constant automatic adjusting method comprising:
 varying a phase of an clock signal to be input by a filter circuit capable of varying time constant so as to output the clock signal; 
 comparing, by a phase comparison circuit, a phase of an output signal of the filter circuit with the phase of the clock signal, and generating output voltage according to a phase difference between the phase of the output signal of the filter circuit and the phase of the clock signal; 
 comparing, by at least three comparators, the output voltage with a plurality of different voltages; 
 counting, by an up-down counter, a number of output bits of either one of at least three different values, each values being set in accordance with an output result of the comparators; and 
 varying, by a control circuit, the time constant of the filter circuit in accordance with the number of output bits counted by the up-down counter. 
 
     
     
       3. A time constant automatic adjusting method, comprising:
 varying a phase of an clock signal to be input by a filter circuit capable of varying time constant so as to output the clock signal; 
 comparing, by a phase comparison circuit, a phase of an output signal of the filter circuit with the phase of the clock signal, and generating output voltage according to a phrase difference between the phase of the output signal of the filter circuit and the phase of the clock signal; 
 comparing, by at least three comparators, the output voltage with a plurality of different voltages; 
 counting, by an up-down counter, a number of output bits of either one of at least three different values, each values being set in accordance with an output result of the comparators; 
 varying, by a control circuit, the time constant of the filter circuit in accordance with the number of output bits counted by the up-down counter; and 
 connecting or disconnecting, by a switch, the clock signal to be input to the filter circuit and the clock signal to be output from the filter circuit to the phase comparison circuit.